1. Field of the invention
The present invention relates to a receiving circuit and a receiving method for receiving a multi-carrier signal in a radio communication system, and particularly, a receiving circuit and a receiving method for receiving a radio signal multiplexed by a multi-band Orthogonal Frequency Division Multiplexing (OFDM).
2. Description of the Related Art
Ultra Wide Band (UWB) communication is proposed as close range and large volumetric radio communication recently. Especially, a proposal of a UWB system for Multi-Band Orthogonal Frequency Division Multiplexing (MB-OFDM) is coming under the spotlight. MB-OFDM is discussed under TG3a of IEEE 802.15 committee in order to standardize. Details of MB-OFDM are described in IEEE P802.15-03/268r1 and IEEE P802.15-03/267r6.
A basic technique about OFDM and MB-OFDM is described below. OFDM is a communication method which is called a multi-carrier communication method. A multi-carrier communication method transmits data using a plurality of carriers. OFDM transmits and receives a plurality of bits using a plurality of carriers. A plurality of carriers are called sub-carriers in OFDM. Center frequencies of any two carriers, which are adjacent with each other, are orthogonal in sub-carriers, and single frequency band of OFDM is occupied with a plurality of sub-carriers.
In a normal OFDM system, each sub-carrier is modulated by using multi-value Quadrature Amplitude Modulation (QAM), so that one sub-carrier can transmits a plurality of bits. Therefore, in case that m sub-carriers are used in OFDM communication and each sub-carrier can transmit n bits, m*n bits of data are transmitted at once in OFDM, data transmitted at once is called one symbol.
Modulation and demodulation in OFDM are described below. FIG. 11 shows a transmitting apparatus and a receiving apparatus which modulate or demodulates data in OFDM system. In OFDM modulation, serial data for transmission is supplied to transmitting apparatus. A serial-parallel converter 1101 of the transmitting apparatus converts serial data to parallel data. This serial-parallel conversion is carried out because OFDM uses a plurality of carriers at once.
Then, sub-carrier modulators 1102 modulate sub-carriers as described above. Multi-value QAM shows a plurality of bits using amplitude and phase. Data of multi-value QAM shown in complex number plane.
Sub-carrier modulated signals are inverse discrete Fourier transformed. This transform is performed in each sub-carrier frequency. Signals, which are inverse discrete Fourier transformed and synthesized by inverse discrete Fourier transformer (IFFT) 1103, are transmitted as an OFDM signal (multi-carrier signals) via D/A converter 1104 and antenna. More processes are performed in order to transmit an OFDM signal, however, these processes are omitted here.
In demodulation of a modulated signal, an inversed operation described above is performed. In a receiving apparatus, received signals are detected and an OFDM signal is taken out. An OFDM signal is converted to a digital signal by an A/D converter 1105. Fourier transformer (FFT) 1106 discrete Fourier transforms a digital signal and separates a signal to a plurality of sub-carrier signals. Then, sub-carrier demodulators 1107 demodulate sub-carrier signals. A parallel-serial converter 1108 converts sub-carrier signals to serial data, and received data is output.
In OFDM method, in order to accurately demodulate a sub-carrier modulated signal, noises that are introduced in transmission path have to be removed by signal processing. In signal processing, a phase rotation amount of a received symbol has to be obtained in order to adjust and remove a phase noise due to fluctuation of a local frequency of a transmitting apparatus and a receiving apparatus. Therefore, several sub-carriers are set as pilot-sub-carriers in OFDM. Pilot-sub-carriers are predetermined sub-carriers, and pilot-sub carriers do not have data. In a receiving apparatus, a phase rotation amount is calculated based on pilot-sub-carriers.
MB-OFDM included in OFDM varies a frequency band occupied by a symbol. For example, assuming that center frequencies of frequency bands occupied by a plurality of sub-carriers correspond to f0, f1 and f2, MB-OFDM changes a center frequency of a frequency band by every symbol, such as f0 to f1, f1 to f2 and f2 to f0. This operation is called frequency hopping. FIG. 12 shows transmitted symbols with frequency hopping. FIG. 12 shows an example of transmitted data D1 to D17 with frequency band hopping, such as BAND1 to BAND2 and BAND2 to BAND3.
In case that a symbol is received in an OFDM system, a phase rotation amount of the received symbol has to be calculated in order to accurately reproduce received information. A phase rotation amount is calculated using pilot-sub-carriers interposed in data sub-carriers. Phase rotation amounts of pilot-sub-carriers can not use directly to adjust a symbol because of unexpected noise or fading. Therefore, in single band OFDM without frequency hopping, calculated phase rotation amounts are smoothed among a plurality of OFDM symbols. Smoothed phase rotation amount is used in order to adjust a received symbol. In this specification, “smoothing phase rotation amounts” means calculated phase amounts are smoothed among a plurality of OFDM symbols.
On the other hands, MB-OFDM performs frequency hopping by every symbol. Therefore, a frequency band of the latest OFDM symbol is different from that of OFDM symbol received before.
Therefore, a receiving circuit and a receiving method that can calculate a phase rotation amount and perform phase tracking are required in MB-OFDM.